Anionic clays possess a crystalline structure consisting of positively charged layers constituting of specific combinations of metallic hydroxides amongst which anions and water molecules are found. These compounds can be represented by the following general formula:[Mn+1−xMq+x(OH)2]Ap−x/p.mH2O where:Mn+ represents a metallic cation,Mq+ represents a metallic cation with upper positive charge (q>n),Ap− represents any anion.
It has been found in nature that many minerals are isomorphs, characterized by having different stoichiometries, with more than one anion or more than two cations, or with small quantities of cations in the brucite-like interlaminar region. Such crystalline structures include pyroaurite, sjogrenite, hydrotalcite, stichtite, reevesite, eardleyite, manasseite, barbertonite, takovite, desautelsite, and hydrocalumite, among others. The chemical formulas of synthetic anionic clay forms include: [Mg6Fe2(OH)16]CO3.4H2O, [Mg6Al2(OH)16]CO3.4H2O, [Mg6Cr2(OH)16]CO3.4H2O, [Ni6Fe2(OH)16]CO3.4H2O, [Ni6Al2(OH)16]CO3.4H2O, [Fe4Fe2(OH)12]CO3.mH2O, [Ca2Al(OH)6][(OH)0.75(CO3)0.125.2.5H2O6]OH.6H2O, [Ca2Al(OH)6]OH.3H2O, [Ca2Al(OH)6]OH.2H2O, [Ca2Al(OH)6]OH, [Ca2Al(OH)6]Cl.2H2O, [Ca2Al(OH)6 ]0.5CO3.2.5H2O, [Ca2Al(OH)6 ]0.5SO4.3H2O, [Ca2Fe(OH)6]0.5SO4.3H2O, [(Ni,Zn)6Al2(OH)16]CO3. 4H2O, [Mg6(Ni,Fe)2(OH)16](OH)2.2H2O, [Mg6Al2(OH)16](OH)2.4H2O, [(Mg3Zn3)Al2(OH)16]CO3.4H2O, [Mg6Al2(OH)16]SO4.m H2O, [Mg6Al2(OH)16](NO3)2. mH2O, [Zn6Al2(OH)16]CO3.mH2O, [Cu6Al2(OH)16]CO3.mH2O, [Cu6Al2(OH)16]SO4. mH2O, [Mn6Al2(OH)16]CO3.mH2O.
In order to understand the structure of these compounds, it is necessary to take the structure of brucite Mg(OH)2 as a reference, where Mg2+ is found octahedrally coordinated to six hydroxyl groups, which, upon sharing their edges, form infinite layers. These layers pile up one on top of the other and are held together by hydrogen bridges. For example, when the Mg2+ is replaced by Al3+, the presence of the aluminum atoms produces positive charges in the structure which are compensated for with interlaminar anions together with water molecules. The most common anions are carbonates, but they can be NO3−, OH−, Cl−, Br−, I−, SO42−, SiO32, CrO42−, BO32−, MnO4−, HGaO32−, HVO42−, ClO3−, ClO4−, IO3−, S2O32−, WO42−, [Fe(CN)6]3−, [Fe(CN)6]4−, (PMo12O40)3−, (PW12O40)3−, V10O266−, Mo7O246−, etc.
Specialists in this field will realize that the anionic clays are commonly referred to as, “Mixed metal hydroxides.” This expression is derived from the fact that, as was noted earlier, the positively charged layers of the metallic hydroxides can contain two or more different metallic cations in different oxidation states, such as, Mg2+, Ni2+, Zn2+, Al3+, Fe3+, Cr3+, etc.
Additionally, and given that the X-ray diffraction patterns of many of the anionic clays are similar to the natural mineral known as hydrotalcite, [Mg6Al2(OH)16](CO3).4H2O, they are commonly called, “Hydrotalcite-like compounds.” This term has been amply used in the scientific article and patent literature for many years. In fact, the terms, “Anionic clays,” “Mixed metal hydroxides,” “Hydrotalcite-like compounds,” and “Double layered hydroxides,” are closely related to each other and are used indistinctly. For the sake of simplicity, the term, “Hydrotalcite-like,” is defined and used in a manner consistent with the literature, given that hydrotalcite, strictly speaking, has been the most studied anionic clay in the last decade.
It is known that anionic clays decompose in a predictable manner, and when they are heated without exceeding certain temperatures the materials resulting from the decomposition can be rehydrated, and optionally re-supplied with various anions different from the one that was originally found in the interlaminar region and from those that were removed during heating, thus reproducing the original anionic clay or a very similar one. The decomposition products of such heating are frequently referred to as “collapsed” or “meta-stable” anionic clays. However, if these collapsed or meta-stable materials are heated to temperatures above 800° C., the decomposition products of said anionic clays will not be able to be rehydrated and/or reconstituted to their original structure. Such anionic clay thermal decomposition process has been studied in detail in the academic and patent literature, for example, Miyata in “Physico-Chemical Properties of Synthetic Hydrotalcites in Relation to Composition”, Clays and Clay Minerals, Vol. 28, 50-56 (1980).
One of the main problems to resolve when multimetallic anionic clays are prepared is proving that the cations really have incorporated themselves into the laminar structure. What's more, depending on the chemical nature of the cation, its velocity and precipitation pH may be different from that of the other cations. If the precipitation velocities amongst them are very different, a phase segregation will be obtained; that is, the cations will not be able to be incorporated in a uniform manner into the sheets of the anionic clay. For this reason, it is difficult to obtain multimetallic anionic clays whose crystallographic phases show themselves to be pure in an ample interval of cation compositions.
The preparation of synthetic anionic clays began with the pioneering works of Feitknecht and Gerber (1942) [Feitnecht, W., Über die Bildung von Doppelhydroxyden zwischen zwei-und dreiwertigen Metallen. Helv Chim. Acta (1942), 25, 555-569 and Gerber M., Zur Kenntnis der Doppelhydroxyde und basische Doppelsalze. III Über Magnesium-Aluminiumdoppelhydroxyde. Helv Chim. Acta (1942) 25, 131-137]. Their research group was the first to synthesize hydrotalcite via coprecipitation of a solution containing both the metallic cations, MgCl2 and AlCl3, with NaOH. Since then, similar syntheses have been described in the literature, all of them based on the precipitation of Mg and Al salts, followed by exhaustive washes to eliminate the remaining excess ions. Later on, new preparation methods were proposed, hydrothermal treatment among them, which was described by G. Mascolo and O. Marino, (1980) [Mascolo G., Marino O., A new synthesis and characterization of magnesium-aluminum hydroxides, Miner. Magazine 1980, 43, 619-621]. This consists in heating a magnesium oxide and alumina gel suspension in a closed container for 7 days. Nevertheless, the final product contained brucite phases, gibbsite, and in some cases boehmite, depending on the heating temperature and on the initial suspension's Mg/Al ratio.
Currently, a large number of patents exist regarding the preparation and use of anionic clays; those that stand out are described below:
In GB Patent No. 1,086,779 (1967) granted to Merck & Co. Inc., the preparation of magnesium aluminum hydroxycarbonates is described, where said hydroxycarbonates are prepared by the contact of a magnesium carbonate slurry, magnesium bicarbonate, or a mixture of these, with a soluble aluminum salt that can be aluminum sulfate, aluminum chloride, or aluminum nitrate, in the absence of sodium ions. The final sample is filtered and washed. It is proposed that these compounds be used as antacids.
With this same purpose for use, Kyowa Chemical Industry, in GB Patent No. 1,185,920 (1970), describes a process for the preparation of hydrotalcite, which encompasses the formation of a mixture at a pH of at least 8 from an aluminum component with a magnesium component, both of which are dissolved in an aqueous environment in the presence of carbonate ions in an Al2O3:MgO ratio of 1:6. The mixture may age between 0-150° C., and the carbonate to aluminum atom ratio should be at least 1/9. The final sample is filtered and washed.
In U.S. Pat. No. 4,447,417 (1984) and U.S. Pat. No. 4,560,545 (1985), by Robert G. W. Spickett, granted to Anphar S. A., the preparation of the bimetallic anionic clay (BAC): Mg6Al2(OH)14(CO3)24H2O is described. The process for preparing this magnesium-aluminum basic carbonate encompasses heating a mixture of aluminum hydroxide and magnesium hydroxide in an aqueous environment that contains ammonia or a soluble nitrogenated organic base; whether it be a mono, di, or trialkylamine that contains more than 4 carbon atoms in the alkylamine radical(s), or pyridine or piperidine, the quantity of ammonia or of the organic base should be at least 6 moles per mole of Al2O3 present, at a temperature between 70-100° C. at atmospheric pressure. A carbon dioxide current is passed through the reaction mixture. The resulting mixture is put to reflux from 1 to 12 hours while the CO2 stream passes through the mixture at a speed high enough to produce the AB.
Klaus Schanz, in U.S. Pat. No. 4,539,195 (1985), granted to Giulini Chemie GmbH, lays claim to the preparation of a crystalline aluminum-magnesium basic carbonate with the formula Al2Mg6(OH)12(CO3)3xH2O (x≧4) and its use as an antacid. The preparation process for this material includes mixing basic magnesium carbonate and at least one compound selected from the magnesium hydroxide and active magnesium oxide in an active aluminum hydroxide aqueous suspension where the magnesium oxide provided by the basic magnesium carbonate is found between 44-70% in weight of the total magnesium oxide, thus obtaining the aluminum-magnesium basic carbonate at temperatures of 50-100° C. as a final product.
Misra Chanakya, in U.S. Pat. No. 4,656,156 (1987), granted to Alcoa, describes the use of hydrotalcite as an anion adsorbent in which anywhere between 20-80% in weight may be the hydrotalcite, and between 80-20% in weight may be an activated alumina; said composition can be activated by heating between 500-600° C. The synthesis is carried out by the reaction of activated magnesia with an aqueous alkaline solution that contains aluminate, carbonate, and hydroxyl ions at a temperature between 80-100° C. The aqueous alkaline solution contains the aluminate, carbonate, and hydroxyl ions understood as NaOH, Na2CO3, and Al2O3.
John Kosin, in U.S. Pat. No. 4,883,533 (1989), granted to J.M Huber Corporation, describes the production of synthetic bimetallic anionic clays that contain phosphates, formula xMgO.Al2O3.yPO4.zH2O, which possess improved characteristics such as flame retardants. The procedure for preparing these synthetic BACs consists in an aqueous system that includes an Mg source, an Al source, and a carbonate source, which react with phosphoric acid to generate the final product. The magnesium sources can be: MgO, Mg(OH)2, MgCO3, and other water soluble Mg salts. The magnesium hydroxide is the preferred reagent in 40-60% of solids. The aluminum must be present in an adequate aluminum salt: the preferred reagents are sodium aluminate as an aqueous solution or solid trihydrated aluminum. Of the reagents that contain carbonates, the following are preferred: alkaline metal carbonates or bicarbonates, CO2, alkaline metal earth bicarbonates, and mixtures of the aforementioned. Notwithstanding all of this, the preferred reagent is Na2CO3. These reagents are mixed in a closed reactor; the mix is heated at 150-200° C. for 1-3 hours, then filtered, washed, and dried.
Misra Chanakya, in U.S. Pat. No. 4,904,457 (1990), granted to Alcoa, describes a method for producing high yields of hydrotalcite that includes the activated magnesia reaction with an aqueous solution that contains aluminate, carbonate, and hydroxyl ions. The method includes a first step in which the carbonate or magnesium hydroxide is heated between 450-850° C. to form an activated magnesia or magnesium oxide. The method is appropriate for producing synthetic hydrotalcites from the Bayer liquid.
Alain A. Schutz, in U.S. Pat. No. 4,970,191 (1990), granted to Aristech Chemical Corporation, lays claim to a method for preparing a catalyst based on basic mixed oxides whose preparation consists of dispersing a pseudoboehmite in a water soluble acid (which may be acetic or nitric), and subsequently adding MgO o Mg(OH)2 with an Mg/Al ratio in the gel of 1:1 up to 10:1. The mixture is stirred until the MgO disappears; the product is dried and calcined between 300-500° C. for 1-24 hours.
Donald Grubbs, in U.S. Pat. No. 5,362,457 (1994), granted to Alcoa, describes a method for producing an intercalated hydrotalcite without the need to form the hydrotalcite in the first place and then having to activate it later before substituting the anions in the hydrotalcite's structure. The invention includes reacting activated magnesia with an aqueous solution of aluminate, carbonate, and hydroxyl ions, as well as the anions that will form the BACs. It is preferable that the aluminate be a sodium aluminate and that the aqueous solution be free from carbonates. The anions that are selected should, preferably, belong to the bromide, chloride, sulfate, borate group, or combinations of these. Furthermore, this method provides a solid with high purity and a large yield.
Alain Schutz, in U.S. Pat. No. 5,399,329 (1995), granted to Aristech Chemical Corp., lays claim to the synthesis of a hydrotalcite-like material that has a “laminar” morphology and a width/thickness ratio of 50:1-5000:1. This material is represented by the following formula: (Mg1−xAlx)(OH)2xA−mH2O, where A− is a monocarboxylic anion in form RCOO−, and R is CnH2n+1 with n=0-5; x is a number between 0.2 and 0.4; and m is a number between 0 and 4. In this process, a mixture of divalent metal cations, mainly made up of magnesium, and trivalent metal cations, mainly made up of aluminum, are reacted with monocarboxylic anions that have anywhere from 1-6 carbon atoms in a ratio of 1:1-10:1 in an aqueous slurry at 40° C. and at a pH of 7-12. The ratio of the monocarboxylic anion/trivalent metal cation is from (0.1-1.2):1. Thereafter, the slurry is dried obtaining a hydrotalcite-like material with laminar morphology and a width/thickness ratio of 50-5000. The Mg sources may be MgO or Mg(OH)2. Another of this invention's innovations is that the Mg and the Al can be substituted up to 50% in mole for divalent cations selected from the Ni, Cu, Zn, Co, and Mn group, and for trivalent cations selected from the Fe and Cr group, respectively.
Martin Edward, in U.S. Pat. No. 5,514,361 (1996), granted to Alcoa, presents a method for preparing a synthetic meixnerite obtained by the combination of magnesium oxide and alumina powder, preferably an alumina with a specific area≧100 m2/g in a carbonate-free environment. The MgO and the alumina are combined in water, heating the mixture between 50-180° C. The solid is separated producing a meixnerite compound. One of the key stages during the synthesis process is to maintain the solution in an inert atmosphere in order to avoid the incorporation of other anions such as carbonates and nitrates, mainly.
The meixnerite synthesis is also claimed in U.S. Pat. No. 5,645,810 (1997), granted to Alcoa. The method consists of calcining a hydrotalcite between 500-900° C., cooling it, and hydrating it in a CO2 free atmosphere. When the meixnerite is produced this way, it generates a solid with a specific area≧290 m2/g.
In a series of patents: U.S. Pat. No. 6,171,991 (2001), U.S. Pat. No. 6,376,405 (2002), U.S. Pat. No. 6,440,887 (2002), U.S. Pat. No. 6,440,888 (2002), U.S. Pat. No. 6,444,188 (2002), U.S. Pat. No. 2003/0049189, U.S. Pat. No. 6,652,828 (2003), U.S. Pat. No. 6,593,265 (2004), U.S. Pat. No. 6,710,004 (2004), U.S. Pat. No. 6,800,578 (2004), and U.S. Pat. No. 6,815,389 (2004) granted to Akzo Nobel N.V., Dennis Stamires and collaborators describe a process for producing anionic clays using economical raw materials by means of a synthesis process adequate for being carried out in a continuous manner. The authors indicate that, due to the nature of the precursors, there is no need to carry out washes or to filter. Likewise, according to the authors, it is possible to obtain an ample variety of M2+/M3+ ratios through the procedure described in these patents. The processes consist mainly of: a) Putting a magnesium source (generally MgO, Mg(OH)2, MgCO3 or their mixtures) in contact with an aluminum source (trihydrated aluminum, gibbsite, bayerite, norstrandite, boehmite, Al(NO3)3.9H2O or their mixtures in an aqueous medium; b) submitting them to a treatment at room pressure and temperature or at elevated pressure and temperatures; c) adjusting the pH with an acid or base and aging the mixture at temperatures between 85-240° C. from 5 minutes to 5 days. In the same manner, the authors indicate that during the process of preparing the anionic clays an anionic exchange can take place with pillared anions, mainly with V10O286−, Mo7O246, tungstates, phosphates, borates, vanadates, and/or their mixtures.
In U.S. Pat. No. 6,440,888 (2002), Stamires describes a process for preparing bimetallic anionic clays (BACs) where the divalent cation can by anything except magnesium. The examples encompass the preparation of BACs: ZnAl, CuAl, and FeAl with molar ratios of M2+/M3+˜2. The reaction times vary from 1-18 h, with temperatures between 50-250° C. The aluminum sources are calcined alumina and gibbsite.
Simultaneously, in U.S. Pat. No. 6,444,188 (2002), U.S. Pat. No. 2003/0049189, and U.S. Pat. No. 6,652,828, Stamires lays claim to the process for obtaining bimetallic anionic clays where the trivalent cation is anything but aluminum. The examples include the preparation of BACs: MgGa, MgCr, and MgFe through the contact of MgO with gallium oxide, gallium nitrate, chromium nitrate, or ferric nitrate maintaining an M2+/M3+=2.3 ratio. The slurry is adjusted to pH=10 with NH3OH. The mixture can be treated from 50-250° C. for 1-18 hours.
William Jones, in U.S. Pat. No. 6,541,409 (2003), granted to Akzo Novel NV, claims an anionic clay production process using a boehmite without peptization. The process involves the reaction of slurry that contains a boehmite without peptization and a magnesium source. In addition, according to the invention there is no need to wash the product.
The anionic clay preparation is carried out through the contact of MgO with the following aluminas: Catapal or Versal V-250 or Condea P200, continuing to stir the slurry for 4-48 hours at 0-185° C.
In the last decades, anionic clays have found multiple applications in fields such as: medicine, support or catalyst for different organic reactions, adsorbents to eliminate or reduce the sulfur oxides (SOx) and/or nitrogen (NOx) in gas streams, flame retardant, etc.
Standing out amongst the applications as a catalyst are W. T. Reichle's pioneering works, protected by U.S. Pat. No. 4,458,026 (1984) and U.S. Pat. No. 4,476,324 (1984) assigned to Union Carbide Co., in which the use of mixed oxide obtained from the calcination of the binary anionic clay MgAl in the conversion of the acetone into mesityl oxide and isophorone, as well as the aldol condensation of other compounds containing carbonyl groups, is described.
In this same sense, the patents by A. A. Schutz, U.S. Pat. No. 5,055,620 (1991) and U.S. Pat. No. 5,202,496 (1993) granted to Aristech Chemical Co., protect the preparation and use of basic mixed oxides with Mg/Al: 1 to 10 ratios, as effective catalysts in the condensation of acetone into isophorone, and other reactions catalyzed by bases such as olefin isomerization and the aldol condensation of aldehydes.
Holmgrem et al., in U.S. Pat. No. 5,254,743 (1993) assigned to UOP, describes the use of solid bases resulting from the calcination of laminar double hydroxides as effective catalysts in aldol condensations of aldehyde and ketone, in particular from the conversion in liquid phase of the n-butyraldehyde into 2-ethyl-2-hexenal with a high yield and good selectivity.
Engel et al, in U.S. Pat. No. 5,350,879 (1994) assigned to UOP, proposes the use of solid solutions resulting from MgAl anionic clay calcination resultants as a basic catalyst in the transesterification of alkyl acetates and their respective alcohols, with an excellent yield and high selectivity.
Given the importance of the heterogeneous basic catalysts in fine chemistry, the easy control of their physicochemical properties, which depend on the metallic cations incorporated into the network, their amount, and the nature of the interlaminar anions, obtaining multimetallic anionic clays through a simple and economically viable method is of great importance. The scientific and patent references repeatedly include the use of acid and/or basic substances, organic or inorganic, in order to adjust the pH of the solutions, which includes NaOH, NaHCO3, Na2CO3, KOH, K2CO3, NH4OH, (NH4)2CO3, or any alkaline compound. Nevertheless, the use of alkaline metal hydroxides or carbonates requires that the final solid be submitted to a series of exhaustive washes with the goal of eliminating these ions.